Discovering Cathodic Biocompatibility for Aqueous Zn–MnO2 Battery: An Integrating Biomass Carbon Strategy

Highlights γ-MnO2 loaded on N-doped biomass carbon from grapefruit peel is firstly developed. The splendid cathodic properties (e.g., coulombic efficiency: ~ 100%, energy density: 553.12 Wh kg−1) are gained. The biomass strategy guaranteed via cytotoxicity test shows a clinical potential. Zn-ion storage efficiency is boosted mainly by regulating Mn–O bond and Mn domains. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-024-01334-3.

The recombination of carbon materials and MnO 2 has been identified as an important way to optimize the cathodic performance of aqueous Zn-MnO 2 batteries [97][98][99].Chen et al. synthesized carbon nanofiber@δ-MnO 2 with a facile method combining solid-grinding and wet-chemical reaction, and achieved a capacity of 277 mAh g −1 and capacity retention of 79.78% after 700 cycles at 200 mA g −1 [100].Li et al. prepared N-doped carbon nanowires incorporated with δ-MnO 2 by hydrothermal method, the discharge capacities of which were 325 mAh g −1 at 100 mA g −1 and 90 mAh g −1 at 2 A g −1 respectively, and its cycle life after 2500 cycles was 95% at 2 A g −1 [101].Huang et al. electrochemically deposited α-MnO 2 onto carbon nanotube as cathode, which achieved the specific capacities of 292.7 and 105.6 mAh g −1 at 0.2 and 3 mA cm −2 respectively, and its cycling life remained 88.5% after 300 cycles at 0.3 mA cm −2 [102].Moreover, Kim et al. reported a carbon-coated α-MnO 2 cathode, which exhibits a discharge capacity of 272 mAh g −1 and cycle life of 69.49% after 50 cycles at 66 mA g −1 [103].Overall, carbon materials used to be compound with MnO 2 are mainly based on the conventional commercial materials such as carbon nanofiber, carbon nanotube, etc.Therefore, the research on how to improve Zn-ion storage performance by constructing Mn-based cathode compounded with carbon materials derived from biomass, especially the inexpensive and renewable biomass waste, is still rare.
In this work, the abundant grapefruit peel as a natural biomass carbon source is adopted to synthesize N-doped carbon carrier powder (CP) through a simple calcination in N 2 atmosphere, and γ-MnO 2 prepared by electro-deposition is uniformly loaded onto CP to obtain the composite cathode (γ-MnO 2 @CP).A systematic study about the improving mechanism of Zn-ion storage efficiency via compounding CP is conducted from multiple perspectives of Jahn-Teller effect, Mn valence, and Mn domains, etc. Besides, the in vitro cytotoxicity experiments of pure γ-MnO 2 and γ-MnO 2 @CP are carried out to investigate the application prospect in the field of biomedicine.Therefore, the above research provides a valuable guidance for the comprehensive utilization of wasted biomass to design high-performance MnO 2 -biomass carbon cathode.

Sample Preparation
The grapefruit peel was washed three times by deionized water, then heated at 80 °C for 3 h and crushed into yellow powder, which was heated at 650 °C for 2 h under a nitrogen atmosphere to obtain CP.A three-electrode system composed of a stainless steel working electrode, a saturated calomel reference electrode, and a platinum plate counter electrode was used for MnO 2 electro-deposition under the current density of 5 mA cm −2 for 30 min with electromagnetic stirring, and the electrolyte was made up of 0.5 M 1 3 Mn(CH 3 COO) 2 and 0.5 M Na 2 SO 4 , the chemical reaction occurred on the surface of stainless steel working electrode is described as: Mn 2+ + 2H 2 O → MnO 2 + 4H + + 2e − .Finally, the thin films of MnO 2 electrochemically deposited on the stainless steel electrode were scrapped off.The mixtures of as-prepared MnO 2 and CP (Quality percentage of CP: 10, 20, 30, and 40 wt%) were magnetically stirred in deionized water for 1 h, and then ultrasonically dispersed for another 1 h respectively, the above solutions were centrifuged and then dried at 80 °C (Heating rate: ~ 2 °C min −1 ) in air for 3 h to obtain the cathodic active materials, which were labelled as CP-10, CP-20, CP-30 and CP-40, respectively.Besides, the as-prepared MnO 2 mentioned above was labelled as CP-0.

Materials Characterization
The X-ray diffraction (XRD) profiles were measured with Bruker-D8 Advance X-ray diffractometer (Cu Kα radiation, 2θ step: 0.02°) and analyzed with Jade 5.0 software.The spectroscopic property was tested though PerkinElmer Spectrum 100 FTIR.The micro morphology was observed through JEOL JEM-2100F TEM and Nova Nano SEM 450.The composition and valence state were tested by Thermo ESCALAB 250XI XPS.

Computational Calculation
All the theoretical calculation about the models of Zn 2+ intercalating into the tunnel structures of pure γ-MnO 2 and γ-MnO 2 @CP were performed with VASP software based on density functional theory (DFT) in this work.The migration behavior of Zn 2+ was computed by mean square displacement (MSD) using the equation , where r i (0) is the initial position of Zn 2+ , r i (t) is the terminal position of Zn 2+ , and M is the number of simulation, then the diffusion coefficient (D) of Zn 2+ was calculated by fitting 6t to MSD as the Einstein relation: D = lim t→∞ MSD 6t [74].The adsorption energy (E ads ) values of Zn 2+ and OH − on the tunnel structural surface of γ-MnO 2 were calculated, and the bader charges of the above two models were also simulated respectively.Meanwhile, Mn-O bond lengths of MnO 6 octahedron in the above two models were calculated for structural analysis.

Electrochemical Properties
The cathodic active materials (CP-0, CP-10, CP-20, CP-30, and CP-40), acetylene black and poly-vinylidene fluoride were mixed together in accordance with the gravimetric ratio of 7:2:1, and N-methyl-2-pyrrolidone was added into the above mixtures to produce black slurries, which were then painted on 5 stainless steel webs (Diameter: ~ 14 mm) and then heated at 80 °C (Heating rate: ~ 2 °C min −1 ) in air for 8 h to obtain the cathodic current collectors (Load quality of active substance: ~ 2 mg cm −2 ).A CR2032 button battery configuration with a Whatman glass-fiber diaphragm (Grade GF/D), Zn anode, and the electrolyte of 2 mol L −1 ZnSO 4 and 0.1 mol L −1 MnSO 4 was used to estimate the cathodic active materials, the prepared button batteries shared the same numbers as CP-0, CP-10, CP-20, CP-30, and CP-40 respectively.The galvanostatic charge/discharge performance was tested using LANHE CT3002A equipment (Voltage: 0.8-1.8V) based on the active material mass.The cyclic voltammetry (CV) curves were recorded using a CHI660E electrochemical work station (Scan rate: 0.1, 0.2, 0.4, 0.6, and 0.8 mV s −1 ), then b value was obtained according to the equation log(i) = log(a) + blog(v) (i: Peak current; v: Scan rate; a and b: Adjustable values), the pseudocapacitive fitting was calculated on the basis of the relational expression i(V) = k 1 v + k 2 v 0.5 (k 1 v: Non-diffusion controlled contribution; k 2 v 0.5 : Diffusion controlled contribution).

In Vitro Cytotoxicity
3T3 mouse embryonic fibroblast cells were cultured in DMEM medium (Gibco, USA) supplemented with 10% fetal bovine serum (FBS, Gibco, USA) and 1% penicillin/ streptomycin (Gibco, USA) at 37 °C.In vitro cell viability was evaluated by CCK-8 assay against 3T3 cells.Briefly, cells were seeded in 96-well plates and incubated with the as-synthesized materials, including CP-0, CP-10, CP-20, CP-30, and CP-40.10 μL CCK-8 solution was added to each well and the absorbance was recorded on a microplate reader (EnSpire, USA) at 490 nm.Cell apoptosis assay was conducted by using an Annexin V-FITC/PI apoptosis detection kit (Beyotime, China).For apoptosis assay, 3T3 cells were seeded into 6-well plates and incubated with the above-mentioned materials for 48 h.Thereafter, cells were stained with Annexin V FITC/PI for 30 min and the apoptosis percentage was detected by a flow cytometer (BD FACS Calibur, USA).Calcein AM/PI Co-staining was performed to detect the live/ dead cells under different treatments.Following incubation cells were stained with calcein AM and PI for 30 min, and the stained cells were imaged by a fluorescence microscope (NIKON, Japan).For the above experiments, cells in PBS were set as the control group.

Microstructure
The preparation detail of CP is schematically illustrated in Fig. 1a, and the N-doped biomass carbon is expected to be obtained.As shown in Fig. 1b, all the samples display five characteristic peaks indexed to (120), ( 131), (300), (160), and (421) crystal planes, which means that γ-MnO 2 is successfully synthesized by electro-deposition corresponding to JCPDS No. 14-0644 [104], and the broad peaks at around 26° belonging to (002) crystal plane of carbon for CP-10, CP-20, CP-30, and CP-40 are also observed [100].Compared with CP-0, a new peak at around 1025 cm −1 corresponding to Mn-O-C bond is observed in the FTIR spectrum of CP-20 (Fig. 1c), which once again demonstrates the successful recombination of CP and γ-MnO 2 for CP-20.The characteristic planes of CP and γ-MnO 2 are also observed in SAED pattern (Fig. 1d), and the HRTEM images of CP-20 indicate that the interplanar spacings of 0.162 and 0.241 nm are well indexed to the lattice planes of (160) and (131) for γ-MnO 2 (Fig. 1e, f), which further confirms the accuracy of the analytical result from Fig. 1b, d. Figure 1g shows a successful loading of γ-MnO 2 nanoparticles on the surface of irregular flower-like CP.The EDS result shows a uniform element distribution of Mn, O, C, and N for CP-20 (Fig. 1h-k), and the XPS result once again affirms the coexistence of Mn, O, C, and N on the surface of CP-20 (Fig. 1l-o).Besides, Fig. S1 certifies the existence of Mn and O elements for CP-0, and Figs.S2, S3, and S4 also reveal the existence of Mn, O, C, and N elements for CP-10, CP-30, and CP-40 respectively, meanwhile the coexistence of Mn 3+ and Mn 4+ is revealed for all the samples.
The Jahn-Teller effect primarily due to high-spin Mn 3+ with (t 2g ) 3 (e g ) 1 electron configuration in 3d orbital usually leads to detrimental structural disorder for MnO 2 , so MnO 2 with a higher Mn valence is promising to exhibit excellent long-term cycling stability [105].Therefore, the compound strategy of γ-MnO 2 with CP is beneficial to reduce the content of Mn 3+ , and it's worth noting that CP-20 exhibits the highest percent of Mn 4+ (93.68%) and the lowest percent of Mn 3+ (6.32%), which foreshadows the weakest Jahn-Teller effect in CP-20 (Figs. 1l and S1a, S2a, S3a, S4a).

Theoretical Calculation
As shown in Fig. 2a, the calculated D values of Zn 2+ in the internal structures of pure γ-MnO 2 and γ-MnO 2 @CP are 70 × 10 -6 and 174 × 10 -6 Å 2 fs −1 respectively, and it can be seen that the CP composite strategy effectively improves Zn 2+ kinetics.The E ads values of Zn 2+ on the tunnel-shaped surface of pure γ-MnO 2 and γ-MnO 2 @CP are − 2.968 and − 2.353 eV respectively, which demonstrates that Zn 2+ is more liable to migrate smoothly inside γ-MnO 2 @CP (Fig. 2b).Bader charge (1.11e) of Zn 2+ and γ-MnO 2 @CP group is less than that (1.323e) of Zn 2+ and pure γ-MnO 2 group, and this implies a more obvious electron transfer tendency and a stronger binding interaction between Zn 2+ and pure γ-MnO 2 , which hinders the diffusion of Zn 2+ (Fig. 2c).Besides, the nano-sized Mn-based cathode effectively reduces the migration time of Zn 2+ according to the equation τ eq = L 2 /2D (τ eq : Diffusion time; L: Material size; D: Diffusion coefficient) [69], so the combination of nanocrystallization for γ-MnO 2 (Fig. 1g) and CP composite strategy would be helpful to boost Zn storage efficiency theoretically.

Electrochemical Property
It is worth noting that the cathodic peaks shift toward higher values and the anodic peaks shift toward lower values for CP-10, CP-20, CP-30, and CP-40 compared with those of CP-0 from the CV curves at 0.4 mV s −1 , which indicates the reduced inherent voltage polarization plausibly related to CP composite, and especially the smallest polarization is seen in CP-20, and CP-20 also shows the highest peak current density implying its largest electrochemical capacity (Fig. 3a).CP-20 with a medium potential of ~ 1.4 V displays a maximum discharge capacity of 391.2 mA g −1 and a minimum voltage hysteresis when the current density is 0.1 A g −1 among the samples (Fig. 3b).The high rate discharge ability of γ-MnO 2 mixed with CP are better 1 3 than that of pure γ-MnO 2 when the current density increases from 0.1 to 5 A g −1 and then reduces to 0.1 A g −1 , besides CP-20 also exhibits the optimal discharge capability of 189.8 mAh g −1 at 5 A g −1 compared with other samples (Fig. 3c).The cyclic stability of CP-0 is only 24.54% after 200 cycling times at 0.1 A g −1 , while CP-20 demonstrates the best cycling performance (89.47%) and coulombic efficiency (~ 100%) (Fig. 3d).In particular, the 3000 times cycling life of CP-20 achieves even high up to 92.17%, and its coulombic efficiency still maintains at ~ 100% at 5 A g −1 (Fig. 3e).Furthermore, the specific energy density of CP-20 at 0.1 A g −1 reaches 553.12 Wh kg −1 , which is superior to that of CP-0 (250.32 Wh kg −1 ), CP-10 (383.08 Wh kg −1 ), CP-30 (427.00Wh kg −1 ) and CP-40 (307.21Wh kg −1 ).More importantly, the energy density of CP-20 in our work is also superior to those reported in literatures (Fig. 3f).To sum up, the recombination strategy with CP is of benefit to the electrochemical performance enhancement for γ-MnO 2 .
As shown in Figs.S5a, S6a, S7a, S8a, and S9a, the phenomenon that the cathodic and anodic peaks of CV curves for all the samples shift toward negative and positive potentials respectively as the scan rate increases reveals a distinct insertion/deinsertion energy storage behavior of Zn 2+ [74].The CP-20 sample, by contrast, has a stronger redox peak and smaller gap between cathodic and anodic peak, which indicates that CP-20 is promising to have faster dynamics, less polarization and better long cycle performance among the samples.The calculated pseudocapacitive proportion results (Figs.S5b, S6b, S7b, S8b, and S9b) show that CP-20 exhibits the maximum pseudocapacitance contribution, and it is again verified that the reinforced pseudocapacitance behavior by CP composite strategy is beneficial to ameliorate the electrochemical characteristics of γ-MnO 2 .Usually, a larger b-value (close to 1.0) is deemed to be advantageous for strengthening pseudocapacitance behavior so as to improve energy storage dynamics, obviously b-value of CP-20 is lager than that of other samples according to Figs.S5c, S6c, S7c, S8c, and S9c, and this also reveals the effectively promoted pseudocapacitance behaviour of CP-20.Furthermore, The fact that the peak1 and peak2 offset values of CP-20 are the smallest demonstrates again its enhanced intercalation pseudocapacitance (Figs.S5d, S6d, S7d, S8d, and S9d), which strongly supports the above fitting conclusions about pseudocapacitive and b-value.More significantly, the presence of pyridinic-N and pyrrolic-N is considered to be responsible for boosting the pseudocapacitance behavior of electrode materials [106], and this might be another key factor to improve the pseudocapacitance characteristics of γ-MnO 2 via CP composite.(Figs.1o and  S2d, S3d, S4d).

Mechanism Analysis
Under the current density of 0.1 A g −1 , the byproduct Zn 4 SO 4 (OH) 6 •4H 2 O (JCPDS No. 44-0673) emerges at the fully discharged (F-D) state and almost completely fades away at the fully charged (F-C) state on the 10th cycle for CP-20, and the reversible reaction is represented by the following equa- , while Zn 4 SO 4 (OH) 6 •4H 2 O always exists at F-D and F-C states for CP-0 (Fig. 4a).Noticeably, the calculated E ads values of OH − with the surfaces of pure γ-MnO 2 and γ-MnO 2 @CP are − 3.166 and − 1.568 eV respectively (Fig. 4b), this obvious difference illustrates that OH − is more liable to be adsorbed on the surface of pure γ-MnO 2 to promote the formation of Zn 4 SO 4 (OH) 6 •4H 2 O, which reconfirms the conclusion from Fig. 4a.Herein, the CP composite strategy effectively inhibits the side effect and ultimately increases the reversibility and efficiency of Znion energy storage.
The O 1 s XPS spectra of CP-0 and CP-20 are fitted into tetravalent Mn-O-Mn bonds, trivalent Mn-OH bonds, and H-O-H bonds for residual water, correspondingly the average Mn valences of CP-0/CP-20 are estimated to be 3.6 +/3.8 + at F-D state and 3.75 +/3.95 + at F-C state on the 10th cycle according to the area contributions of Mn-O-Mn and Mn-OH components respectively (Fig. 4c).More precisely, the average oxidation state (AOS) of Mn can be computed based on the equation AOS = 8.95-1.13ΔE Mn3s , where ΔE Mn3s is the energy difference between the main and satellite peaks in Mn 3s XPS spectra [107], so the AOS of Mn in CP-0 (F-D), CP-0 (F-C), CP-20 (F-D), and CP-20 (F-C) on the 10th cycle are also 3.6 +, 3.75 +, 3.8 +, and 3.95 + respectively (Fig. 4d), thus the calculated Mn valence from Mn 3s and O 1s XPS spectra are entirely the same, which further illustrates that the CP composite is beneficial to suppress the Jahn-Teller effect by regulating Mn valence, besides the Zn 3p peak of CP-0 is also lower than that of CP-20.Meanwhile, the Zn 2p peaks of CP-20 are apparently stronger than that of CP-0 on the 10th cycle at F-D state (Fig. 4e), which reconfirms the conclusion in Fig. 4d.Different from the Mn-O bonds of γ-MnO 2 @CP nearly without distortion, the Jahn-Teller effect induces a geometric distortion with two longer (O5 and O6) Mn-O bonds of pure γ-MnO 2 according to theoretical calculation (Fig. 4f).The formation of Mn domains is supposed to disrupt the cooperativity of Jahn-Teller effect as schematically shown in Fig. 4g [108], and the Mn domains with different orientation are also visually identified in CP-20 (Fig. 4h) but not in CP-0 (Fig. 4i), hence the CP composite strategy promotes the anisotropic Jahn-Teller distortion to improve the structural stability of γ-MnO 2 .

In Vitro Cell Toxicity
To investigate the intrinsic cytotoxicity of the above-mentioned samples against 3T3 cell, CCK-8 assay is performed to determine the relative cell viability after 24, 48, and 72 h of incubation, respectively.Currently, cells treated with PBS are set as a reference.Negligible proliferation inhibition is observed in cells with the treatment of CP-10, CP-20, CP-30, and CP-40 even up to 72 h of incubation compared with the control group, while exposure to CP-0 exhibits notable cytotoxicity to cells as the relative cell viability decreases over 50% after 72 h (Fig. 5a), indicating the desirable  biocompatibility of the above samples except CP-0.Besides, Calcein AM/PI staining is carried out to assess the live/dead cell after different treatments, and no evident cell death is noticed in cells under these above treatments other than CP-0 as expected (Fig. 5b), which is consistent with the results of CCK-8 assay.Furthermore, flow cytometry is performed to investigate the apoptosis profile of 3T3 cells incubated with each sample for 48 h, and no apoptosis is induced by most of the as-synthesized materials with a total apoptotic cell rate less than 5% compared to the control group, but exposure to CP-0 results in severe cell apoptosis as indicated by nearly 40% of total apoptosis percentage (Fig. 5c), and it is in good accordance with the cell toxicity results.It is verified that CP-10, CP-20, CP-30, and CP-40 possess better biosafety compared with CP-0, which might be attributed to the loading of biocompatible CP.Therefore, the CP composite strategy shows a great potential not only in the field of large-scale energy storage, but also in clinical applications.

Conclusion
The environmental friendly AZIBs with considerable theoretical capacity (820 mAh g −1 ) and appropriate redox potential (− 0.763 V versus standard hydrogen electrode) attract researchers' broad concern recently [109][110][111][112][113][114][115][116][117].Thereinto, biocompatible AZIBs are proposed as candidates for powering biocompatible electronics due to their excellent features of low cost, high-level safety and high-performance [118][119][120].Hence, the biomass CP derived from waste grapefruit peel is successfully prepared, and the electrochemical properties and biocompatibility for the composite cathode of γ-MnO 2 loaded on CP are simultaneously investigated in the present work.The considerable electrochemical properties of 3000 times long cycle stability at 5 A g −1 (92.17%), energy density (553.12Wh kg −1 ), and coulombic efficiency (~ 100%) for the composite cathode with CP quality percentage of 20 wt% are achieved, which is mainly ascribed to the effective regulation of Mn-O bond distance, Mn valence, and Mn domains combined with experimental and DFT computational analysis.Furthermore, the cathodic biosafety is also verified via in vitro test extensively.In brief, this work not only brings forward a feasible countermeasure for structural regulation of multi-function Mn-based cathode with inexpensive biomass-derived carbon, but also paves a novel way for the application of AZIBs in biomedical field.

Fig. 1 a
Fig. 1 a Schematic diagram for the preparation of CP. b XRD patterns of CP-0, CP-10, CP-20, CP-30, and CP-40.c FTIR spectra of CP-0 and CP-20.d SAED, e and f HRTEM, g SEM, h-k EDS of CP-20.XPS high-resolution patterns of l Mn 2p, m O 1s, n C 1s, and o N 1s of CP-20.

Fig. 3
Fig. 3 Electrochemical test results of a CV curves, b constant current charge-discharge profiles, c high rate discharge ability, d 200 times cycling performance, and e 3000 times cyclic stability of CP-0, CP-10, CP-20, CP-30, and CP-40.f Comparison diagram about the energy density calculated based on cathodic active material mass between literatures and CP-20

Fig. 4 aFig. 5 a
Fig. 4 a Ex-situ XRD of CP-0 and CP-20 at F-D/F-C states on the 10th cycle.b Sketch map at F-D state and the calculated E ads values of OH − to the surface of γ-MnO 2 .XPS high-resolution patterns of c O 1s, d Mn 3s and e Zn 2p of CP-0 and CP-20.f Calculated Mn-O bonding distances of MnO 6 octahedra of pure γ-MnO 2 and γ-MnO 2 @CP.g Schematic Mn domains and h, i actually observed Mn domains